This invention relates to an erasable optical recording medium capable of erasing previously recorded information or recording new information while erasing previously recorded information, and a method of optical recording and erasing using the same medium, and more particularly to an optical recording medium capable of sufficiently recording information and erasing the recorded information with a light source of small energy and a method of optical recording and erasing using the same medium.
The quantity of information to be recorded and reproduced has been tremendously increasing in recent years, and there is a keen demand for an innovative method capable of recording and reproducing at higher speed and higher density as compared with conventional information recording and reproducing methods. In this regard, current attention has been directed to various methods of recording and reproducing the information optically. Typical examples of hitherto proposed methods in this field are as follows.
As a first example, a recording method reported by D. Maydan, "The Bell System Technical Journal," Vol. 50, No. 6 (1971), pp. 1761-1788, involves the use of a thin film (about 0.1 .mu.m) of a low melting point metal such as In or Bi formed on a glass or plastic substrate. When a laser beam is applied to the metal thin film in very tiny spots (about 10 .mu.m or smaller in diameter), tiny cavities are formed in the illuminated spots through the absorption of light, the rise in temperature and resultant fusion, and displacement and/or evaporation of the metal. Thus, the illuminated spots become transparent or translucent, so that an optically reproducible and permanent image is formed.
As a second example, U.S. Pat. No. 3,636,526 proposes the use of an amorphous semiconductive material, more specifically a calcogenide not containing oxygen, as typified by Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2, As.sub.2 S.sub.3 and As.sub.20 Se.sub.60 Ge.sub.20. When light such as a laser beam is applied to a thin film of such material, there occurs a change in the bonding state of the atoms of the material as a result of a temperature increase by absorption of light, so that the thin film exhibits a change in the optical density in the illuminated area. By this change in optical density, the information is recorded.
As a third example, U.S. Pat. No. 3,983,542 discloses facilitating the transfer from a low optical density state (an amorphous state) to a high optical density state (a crystalline state) by adding halogen, iodine, bromine, oxygen, steam, alkaline metal element, sulfur, selenium, tellurium or others to a base recording material.
As a fourth example, U.S. Pat. No. 3,971,874 proposes a recording material having a thin film of which essential material is represented by [TeO.sub.x ] were x is smaller than 2.0. When irradiated with light such as a laser beam, the thin film undergoes a change from a low optical density state to a high optical density state as a result of a temperature increase by absorption of light energy. By this change in state, the information is recorded.
As a fifth example, U.S. Pat. No. 4,278,734 proposes the inclusion of S and/or Se by up to 50 mol % in a thin film made of a material represented by [TeO.sub.x ], where x is smaller than 2.0 or [BiO.sub.x ], where x is smaller than 1.5. When this thin film is irradiated with light, the optical density of the film changes, and the information is recorded. The recorded information may be erased by irradiating with light of a sufficient energy density.